1. Field of the Invention
The invention relates to a roll for coating machines for the direct or indirect application of a liquid or pasty medium to one or both sides of a moving web, in particular one of paper or board.
2. Description of the Related Art
A known roll for coating machines for the direct or indirect application of a liquid or pasty medium to one or both sides of a moving web has either a hollow cylindrical or a solid core and is made of metal or fibre-reinforced plastic. The outer circumference of the core is provided with an elastomeric layer, formed of rubber, polyurethane or similar material. Such a roll is assigned a suitable applicator unit and is employed in the on-line or off-line coating machine during papermaking or paper conversion.
In the case of direct application, the roll constitutes an opposing element for supporting and guiding the moving web. In this case, reference should be made to U.S. Pat. No. 3,701,335. A coating roll interacting with a doctor rod includes a metallic core, which has two elastomer outer layers of different thickness and hardness. These layers are incompressible.
In the indirect application process, on the other hand, the liquid or pasty medium is applied to the surface of the roll (applicator roll), the roll subsequently transferring the medium to the web in a press nip through which the web runs. In practice, this apparatus is referred to, for example, as a film press, speedsizer or speedcoater.
In this case, too, rubber-covered applicator rolls or those coated with polyurethane or similar material are used for this purpose. These so-called solid elastomer rolls are pressed against each other with a defined loading, as already mentioned, for the purpose of forming a nip and as a result are flattened, so that the result is a nip broadened by an amount “a”. As a result of this broadened nip, improved transfer of the medium is achieved.
Such a broad nip advantageously also leads to a certain dewatering of the liquid medium and therefore for the most part permits higher coating applications than in the case of a “sharper” nip.
However, the disadvantage of the solid elastomer which outweighs the advantages, is that because of its incompressibility (constancy of volume), in particular upstream and downstream of the nip, a bead is formed as a consequence of the displacement of the elastomeric material. The same negative effect also occurs when a doctor element is pressed against the roll which is formed as an opposing roll and guides the web.
In this case, the bead formation upstream of the nip is greater than that downstream of the nip and, as a result, can even damage the web running into the nip.